FR: Support output configuration across different systems with differently-named outputs

I'm trying to share a dotfiles repo between two systems with mako (plugged into the same monitors but different outputs, so they have different names), and am wondering how to specify the `output` parameter. https://github.com/emersion/mako/pull/228/ mentions the possibility of going down a list of `output`s until one is available which would work. Another more heavy-handed approach would be to include other config files.

Any opinions either way? I could try my hand at this PR.